It is well-known that cryogenic liquid storage vessels may be insulated by using a double walled construction with insulation material disposed therebetween. Granular or particulate type of insulation material, for example, perlite powder, is generally a more effective insulation at high vacuum (i.e., below about 100 microns of mercury) and is more ecomonical to use. In cryogenic transport applications in order to minimize weight, the double walled container is designed with a thin outer shell reinforced by axially-spaced internal support members or rings. The presence of these internal support members within the intermediate evacuable space, however, makes it difficult to fill the space with any insulation that is not granular. Granular insulation is particularly useful because it can simply be poured into the insulation space in such manner that all of the space, even that around the supports, are readily filled.
However, a disadvantage associated with the use of granular or particulate type insulation is its tendency to settle and compact. Settling causes loss of insulation from the upper sections of a storage vessel, thereby increasing overall heat transfer to the cryogenic liquid stored within the vessel. This effect is accelerated when the storage vessel is subjected to outside forces, such as vibration which is normally encountered in mobile applications. Another contributor to the settling of granular insulation in cryogenic storage vessels is thermal stress induced by thermal cycling; that is, the expansion and contraction of the inner storage vessel caused by normal use of the vessel.
Perlite insulation is generally considered a low density materal; however, as compared with some fiberglass insulation, it has a relatively high density. This is a particularly important consideration in mobile transport applications, where the total over-the-road vehicle weight may be limited by regulations. The weight of the unfilled vehicle should therefore be minimized in order to maximize the pay load. This is difficult to do with perlite insulation since relative to some fiberglass insulation it not only has a high initial fill density, but its tendency to settle, necessitates the addition of even more perlite.
Linsay in U.S. Pat. No. 1,730,153 discloses a method for insulating a double-walled tank with fibrous insulation where metallic bands having circumferentially spaced blocks (e.g. of wood) attached thereto are wrapped at selected intervals around a cylindrical inner vessel. The fibrous insulating material (e.g., kapok fiber) is then wrapped around the inner vessel, totally enclosing the spaced, block-equipped bands. The outer shell is then wrapped around the insulated inner vessel and is anchored to the blocks on the spaced bands, e.g., by screws passing through the shell. This construction causes severe compression of the insulation over the blocks in order that they may function as structural members and transmit the shell load without further deformation. Moreover, in a long vessel the shell must be assembled piecemeal since there is no space to permit telescoping of the inner vessel into a unitized outer shell. Consequently, this technique is very labor-intensive and expensive.
Schultz, et. al, in their U.S. Pat. Nos. 4,104,783 and 4,168,014 describe respectively, a method for insulation and an insulation system for cryogenic transport intended as a replacement for the conventionally used perlite insulation. These patents disclose a method for, and an insulation system whereby fiber glass insulation is compressively wrapped around an inner storage container. The insulation is squeezed to a lesser thickness to increase the density of the insulation and to allow the inner vessel to be telescopingly positioned within the outer shell. The squeezing is accomplished by wrapping each layer of insulation with a continuous wire mesh. Tension is maintained on the wire mesh by a rod which is withdrawn once the inner vessel is positioned within the outer shell. Once the rod is removed, the tension is relaxed, and the insulation expands to fill the space between the outer shell and the inner vessel. However, in order to utilize this procedure the fiberglass insulation employed must be compressed to a relatively high density (4 to 6 cubic lbs/cubic feet). This insulation system is disadvantageous because of the aforementioned weight considerations.
Moreover, the above installation approach cannot easily be adapted to the conventionally designed over-the-road cryogenic storage vessels which employ a thin outer shell with internal support rings. Applying all of the insulation to the inner container will leave the spaces between the axially-spaced rings at least partially devoid of insulation. Additionally, the compressed insulation will not be able to expand in the vicinity of the rings resulting in a further increase in the solid conductivity of the insulated apparatus.